Cantide is a 20-mer antisense phosphorothioate oligonucleotide that inhibits telomerase catalytic subunit hTERT, pharmacologic results showed that it had promising antitumor activity. In order to study the pharmacokinetic properties of Cantide, a capillary gel electrophoretic (CGE) method with internal standard was used for the determination of Cantide in rat plasma. Cantide and the internal standard had approximately equal percentage of base composition. Extraction of the phosphorothioate oligonucleotides from plasma was accomplished using two solid-phase extraction columns, a strong anion-exchange column to remove plasma proteins and lipids, followed by a reversed-phase column to remove plasma salts. A second desalting step, achieved by dialysis utilizing a membrane, was required to remove residual ionic material from the extracted sample. The size of the capillary column was 31 cm x 100 microm i.d. with an effective length of 20 cm. The running buffer was a mixture of Tris-boric acid-urea (pH 8.5). The calibration curve was linear in the range of 12.5 - 400 mg/L, with correlation coefficient (r) of 0. 999 8. Intra-day and inter-day relative standard deviations (RSDs) for the extracted samples were 0.398% - 2.46% and 2.75% - 6.07%, respectively. The range of recoveries was 99.53% - 102.1%. The results demonstrate the high accuracy, stability and reproducibility of the procedure.